Chlorine-based disinfectants have played an important role in medical history. As early as the 18th century, the bleaching and disinfecting properties of chlorine were recognized. In 1846, the efficacy was demonstrated of solutions prepared from chloride of lime for prevention of puerperal fever. Shortly thereafter, calcium hypochlorite was used for treating water. World War I prompted investigation of the use of chlorine solutions for cleansing infected wounds. In 1915, Dakin reported experiments concerning the chlorine solution which bears his name (Dakin, H. D.: The antiseptic action of hypochlorites, Brit. Med. J. ii: 809 Dec. 1915). Subsequently, the results of lavaging foul wounds with Dakin's solution were lauded.
Bacteriological studies in this century investigated vital parameters of chlorine solutions. Germicidal action depends on the concentration of hypochlorous acid (Charlton, D. & M. Levine: Germicidal properties of chlorine compounds, Iowa State College Bulletin 35:48, 1937). Both the quantity of chlorine compounds and the pH of the solution determine the concentration of hypochlorous acid. The higher the pH of a chlorinated water solution, the lower the concentration of undissociated hypochlorous acid. At pH 7.5 approximately 50% of the chlorine concentration will exist as undissociated hypochlorous acid (HOCl) while the other 50% will be present as hypochlorite ion (OCl) (White, G.C.: Handbook of Chlorination, N.Y., Van Nostrand Reinhold, 186, 1972). Maximum efficacy of a given concentration of chlorine in solution occurs below pH 5, when all of the chlorine present exists as undissociated hypochlorous acid. Bacteriological tests substantiate that, as hypochlorite solutions are acidified, a marked increase in germicidal action occurs. With pH control, chlorine has proved to be one of the most potent and reliable germicides. As such, it became widely used in water and sewage treatment.
Early studies elucidated a number of problems with chlorine distinfectants. Dakin (previously identified) recognized the "highly irritating character" of some chlorine solutions and learned that tissue irritation could be reduced if both the concentration of hypochlorites and the pH were controlled. Dakin discussed the importance of neutralizing the solution by adding acid to the alkaline hypochlorite compound. Dakin's "neutralized" solution could be continuously applied to wounds without serious irritation.
Another problem is that chlorine solutions are very unstable. The effectiveness of these solutions deteriorates significantly within a few hours. This problem depends largely on the pH of the solution. The more alkaline a hypochlorite solution is, the less hypochlorous acid is present, the less effective it is as a sterilant, and the more stable it is. An example of such an extremely stable hypochlorite solution is ordinary household bleach. The more acid such a hypochlorite solution is, the greater is its hypochlorous acid content, the more effective it is as a sterilant, and the greater the instability of the solution. Cullen indicated that the lower limit of pH of Dakin's solution was about 9, for below that level the solution became too unstable for clinical use (Cullen, G. E. & H. D. Taylor: Relative irritant properties of the chlorine groups of antiseptics, J. Exp. Med. 28:681, 1918). Depending on the initial concentration of hypochlorite, the effective halflife of an acid solution may be only a few hours. A truly neutralized chlorine solution must be mixed immediately prior to use.
The high reactivity of chlorine creates another major problem with its use as a sterilant. Early in this century, Carrel recognized a substantial reduction in the germicidal efficacy of chlorine solutions to which blood serum had been added and remarked about the rapid disappearance of hypochlorite in contact with body tissues and fluids (Carrel, A. & G. Dehelly: The treatment of infected wounds, London, Univ. of London Press, 1918). In a more recent study, addition of only 300 ppm of lactose, a natural sugar, was found to significantly reduce the efficacy of a chlorine solution (Rudolf, A. S. & M. Levine: Factors affecting the germicidal efficacy of hypochlorite solutions, Iowa State College Bulletin, 40:35, 1941). Chlorine readily combines with a wide variety of organic and inorganic substances and is thereby inactivated. Reliable use of a chlorine-based germicide requires adequate cleansing of materials to be sterilized to remove such ubiquitous contaminants as protein or grease. That cleansing must be performed with compatible agents. Most common cleansing agents are not compatible because they bind and inactivate chlorine compounds.
Chlorine solutions have no inherent wetting or detergent capacity. Therefore, microorganisms which are encased in an air bubble or in an oil drop escape destruction. Addition of a surface active or wetting agent to the chlorine solution can increase penetration and contact. Careful evaluation is required to assure that the agent used is compatible and does not bind and inactivate significant amounts of chlorine. Addition of specific compatible detergents to chlorine solutions has been reported to increase their germicidal efficacy. (Petroff, S. A. & P. Schain: The enhancement of bactericidal properties of well known antiseptics by addition of detergents, Quart. B. Seaview Hosp. 5:378-379, 1940). Such combinations of chlorine compounds with compatible surfactants, though fine germicides, are not now generally used as sterilants because of their corrosive character.
The most difficult to solve problem of chlorine-based germicides is that they are highly corrosive. Concentrations of hypochlorous acid sufficient to sterilize standard bacteriological challenges also quickly attack metals, even stainless steel, causing discoloration and pitting. Metal instruments soaked in chlorine solutions tend to be irreversibly damaged. Sharp edges are destroyed and metal surfaces become pitted and darkened. Instrument manufacturers have been known to remark that they dislike having a product exposed to a chemical which magnifies any metallurgical flaw in even the best stainless steel. Corrosion inhibitors have been recommended for use with hypochlorites (Botham, G. H. & G. A. Dummett: Corrosion by commercial sodium hypochlorite and its inhibition, J. Dairy Res. 16:37, 1949). Sodium silicate was found to be effective in an alkaline solution containing 150 ppm available chlorine. The increase of alkalinity, however, substantially decreases germicidal efficacy. Sodium silicate insufficiently retards corrosion in more concentrated hypochlorite solutions. Many other anticorrosive agents are incompatible because they react with and inactivate chlorine. Resolution of this problem is required in order to provide a chlorine sterilant which is safe for instruments.
Dr. J. C. Kelsey investigated liquid disinfectants for many years. He developed the "Kelsey-Sykes" test, which is now the British referee bacteriological challenge test. In recent years, Dr. Kelsey recommended sodium hypochlorite solutions as the most efficacious liquid disinfectants. He tried to develop an optimal chlorine solution and published a report of his work in 1974 (J. Clin. Path. 27, 632-638). The chlorine solution which he recommends in that article is highly toxic because of its methanol content and its unadjusted alkaline pH. It is also extremely corrosive.
Since Dr. Kelsey retired, Dr. David Coates has carried on the work on hypochlorite solutions. He published a report of his work in 1978 (J. Clin. Path. 31, 148-152). Further experiments indicated that the hypochlorite solutions which he had reported in that publication, were too corrosive for routine use. Therefore, to control the corrosive and tissue-irritating properties of the solution, Dr. Coates reduced the concentrations of alcohol and of available chlorine (to about 200 ppm). He has also learned that buffering the pH of the solution is vital. The chlorine solution which he has most recently recommended, however, contains neither a satisfactory surfactant nor an anticorrosive agent.
Despite the long history of efficacious use of chlorine compounds as disinfectants, few chlorine compounds today are used as sterilants in medical practice. None of the existing solutions meets the requirements of an optimal chlorine-based sterilizing system. The ideal characteristics of such a system are the following:
1. It must reliably kill the standard U.S. Government bacteriological challenge (called the "AOAC" test), preferably within a contact time of 30 minutes or less. Existing liquid sterilants require contact times of many hours to pass the "AOAC" test. PA1 2. It must be completely non-corrosive and non-damaging within the recommended contact time. PA1 3. Its pH must be adjusted to and maintained at approximately neutral in order to increase efficacy, reduce toxicity, and avoid the rapid deterioration of potency which occurs in more highly acid chlorine solutions. PA1 4. It must assure proper preparation of materials to be sterilized, in a way which avoids contamination by organic material or chemicals which might inactivate the chlorine. PA1 5. The system must be so simple that even untrained personnel can prepare and use it without significant error.
In order to maintain the pH of a solution within a desired range, buffers are usually used. One of the most common buffers in the neutral range contains phosphates. For buffering purposes, less than 1% of phosphates will suffice. A solution containing 0.2% hypochlorite, buffered at pH 7.5 with phosphates, is an effiacious sterilant. Such a solution, however, is corrosive and has undesirable surface tension characteristics.
Many investigators have used standard concentrations of phosphates to buffer chlorine solutions (e.g. Friberg, L. & E. Hammarstrom: The action of available chlorine on bacteria and bacterial viruses, Acta. Pathl. Microbiol. Scand., 38:128, 1956). Phosphates are included in lists of corrosion inhibitors (Uhlig, H. H: The Corrosion Handbook, NY, Wiley & Sons, 906-7 and 913-14, 1948). Concentrations of phosphates under 1% have been added to chlorine-based disinfectants to reduce corrosion (Diversol EX, Brit. Pat. No. 781,708). However, it has not heretofore been known to use concentrations of phosphates over 1% to achieve substantial corrosion control.
According to the present invention, it has been determined that concentrations of phosphates in excess of 1% markedly reduce the corrosiveness of the solution. The concentration of phosphates necessary to control corrosion depends on several factors including the concentration of hypochlorite, the pH, the presence of a surface active agent, the contact time, and the type of steel. In a solution at pH 7.5 containing 0.02% sodium hypochlorite and a compatible surface active agent, addition of a total of 1.17% phosphates produces a 93% reduction in the corrosion of carbon steel during 4 hours contact. In a similar solution containing 0.2% sodium hypochlorite, addition of a total of 2.75% phosphates produced a 97% reduction in the corrosion of carbon steel during 2 hours contact. When the concentration of total phosphates in the latter solution was increased to approximately 7%, no notable corrosion of common steel occurred. In this instance, even 24 hours of soaking of common steel produced no visible change.
The combination of phosphates in the solution acts as a buffer, resisting alteration in the pH. The concentrations of acid and alkaline phosphates are adjusted to produce approximate neutrality. Under this circumstance, a solution containing as high a hypochlorite concentration as 0.2% exhibits little tendency to tissue irritation. When that solution is repeatedly dropped into the eyes of rabbits, according to directions for the Draize test, no evidence of inflammation occurs (see Example IX).
According to the present invention a surface active agent is included in the chlorine solution. Air bubbles on material not only prevent sterilization but also promote corrosion. The addition of a detergent reduces surface tension and helps avoid formation of bubbles. Few detergents, however, are compatible with both chlorine and phosphates and also effective at pH 7.5. Some effective surfactants which do not react with chlorine, for example, combine with the phosphates causing precipitation. A nonionic agent which meets the desired requirements is a 12 carbon alkyldimethyl amine oxide, for example, dimethyl lauryl amine oxide (see U.S. Pat. No. 3,296,145). This amine oxide is available under the following trademarks: Ammonyx LO and Barlox 12.
The addition of this amine oxide to the sterilizing system of the present invention increases the efficacy of the solution. Suture loops placed on the surface of a buffered chlorine solution lacking detergent will float. If the loops are forced under the surface, many tiny bubbles are visible on their surfaces. When a solution contains an adequate surfactant, the loops rapidly sink into the solution without visible bubbles. With the addition of the amine oxide, solutions of the present invention exhibit adequate surface tension characteristics (see Example VIII).
Inclusion of the surfactant in the sterilizing system of the present invention also measurably enhances corrosion inhibition. For example, when approximately 5% phosphates are added to a solution containing 0.2% hypochlorite at pH 7.5, an 85% reduction in corrosion occurs. When dimethyl lauryl amine oxide is then added to that combination, corrosion virtually ceases. Some surface active agents are listed among corrosion inhibitors (Uhlig, H. H.: The Corrosion Handbook, N.Y., Wiley & Sons, 910-911). However, no previous report is known of potentiation of corrosion inhibition by a combination of phosphates and a surface active spent added to a chlorine-based sterilizing system.
In order to avoid inactivation of the germicidal solution by contaminants such as inorganic salts, organic material or incompatible detergents, and assure thorough wetting of the instrument to be sterilized, immersion in a conditioning bath may be added to the sterilizing system. The conditioning bath consists of ample quantities of separate washing composition containing the same amine oxide as well as agents to control corrosion and pH. Thus, all materials to be sterilized would be preconditioned in a solution containing the conditioning composition. Transfer of small amounts of the conditioning solution to the sterilizing solution will not counteract the efficacy of the latter.
Shelf-life requirements necessitate that the major components of the sterilizing solution (hypochlorite, phosphates and surfactant) remain separate until mixed for use. To avoid errors in preparation, a compartmented bag was devised which assures complete mixing of all compounds when the bat is opened. A colored indicator added to the component in the center compartment acts as a safeguard. Alteration in the central color of the package warns the user that leakage between compartments has occurred. The indicator color immediately disappears when combined with small quantities of hypochlorite. To complete the sterilizing solution, the three compartments in he package are added to a measured amount of water.
Bacteriological tests on a specific formulation of the sterilization system, Solution O (see Examples V-VII) prove it to be a remarkable germicide. High titres of vegetative organisms are killed within minutes. The solution readily passed Great Britain's referee challenge, the "Kelsey-Sykes" test (Kelsey, J. C. & I. M. Maurer: An improved (1974) Kelsey-Sykes test for disinfectants. Pharm. J., Nov. 30, 1974). The sterilization system of the present invention can reliably pass within 30 minutes the American "AOAC Sporicidal Test" which often represents the most difficult bacteriological hurdle. Although some decay in available chlorine occurs during the 24 hours useful life of the solution, that change is insufficient to affect germicidal potency. A 24-hour old solution also passes the "AOAC Sporicidal Test".
Accordingly, an object of the present invention is to provide a sterilization system which is efficacious as a sterilant, which is non-corrosive and non-damaging, which has no deleterious effects on instruments within the recommended contact time, has little tissue irritating properties, has good wetting capacity, and which is stable for more than twenty-four hours.
Another object is to provide a reliable liquid sterilant system operable within a short enough cycle for practical application.
Another object is to provide a liquid sterilant of increased efficacy, reduced toxicity and prolonged life in which the potency does not rapidly deteriorate.
Another object is to provide a sterilization system which does not become contaminated by the materials being sterilized.
A further object is to provide a sterilization system which is so simple that even untrained personnel can prepare and use it without significant chance of error.